The present disclosure generally relates to coreless induction furnaces and relates more particularly to an improved mounting or loading arrangement for an induction coil in a coreless induction furnace. In one embodiment, the coreless induction furnace includes a crucible, an induction coil wound about the crucible, a frame supporting the crucible and the induction coil, and an improved induction coil loading arrangement including at least one clamping assembly for providing a leveraged axial force to an upper side of the induction coil. The improved induction coil loading arrangement will be described with particular reference to this embodiment, but it is to be appreciated that it is also amenable to like applications.
A typical problem faced by designers of coreless furnaces is how to secure the power and cooling coils of the coreless furnace. It is well known that vibration must be controlled when designing the assembly of a coreless furnace coil. If not, mechanical and electromotive forces causing heavy vibrations can lead to premature failure of the coreless furnace coil. By way of example, forces on a single coil of a coreless furnace often reach 2,500 pounds and can sometimes be as large as 5,000 pounds.
One electromagnetic force encountered in coreless furnaces is a compressive force on the coreless furnace's coil that goes to a maximum and returns to zero on each electrical cycle. A typical furnace operating at 300 Hz would have over 1,000,000 cycles per hour or about 12,000,000 cycles if operated for about one half day. For typical fatigue applications, 10-20 million cycles is considered large and, in the case of a conventional coreless furnace, would be met in a day or so of operation.
A common method of reducing fatigue on a member is to retain the member, or the coil in a case of a coreless furnace, at a level so that it does not change state, i.e., a stress going from negative (i.e., compression) to positive (i.e., tension) and to minimize the variation of that stress. For a coreless furnace coil, a force is applied axially to the coil of sufficient magnitude such that the stress on the coil does not return to “zero” and thus the coil is always maintained in compression. Prior art coreless furnace designs applied force directly to the coil utilizing shunts which, generally, are not rigid and are only retained radially to the frame (i.e., not axially). In some limited coreless furnace designs, the coil is retained axially, i.e., from the top and the bottom, but it is generally still free to move relative to the furnace's refractory or the furnace proper.
One conventional means of clamping the coreless furnace's coil was by applying a constant upward force on the power and cooling coils. The clamp applying such a force included a spring-loaded lever mounted near a floor of the furnace for providing a constant upward positive force on the coil. While this conventional means does initially provide the desired positive force on the coil, it is subsequently compromised when the furnace's refractory, which is located above the top of the coil, begins to lift and warp due to the heat and vibration that occurs during operation of the furnace. As a result, operators soon find that they constantly must adjust the set-up torque of the clamp. This can lead to a further problem. That is, when adjusting the set-up torque, over adjustment (e.g., applying too great of a torque during adjustment) can cause lifting of the upper furnace refractory resulting in an impossible condition where the correct positive clamping force cannot be achieved. Due to the afore-mentioned drawbacks, the power level of the coreless furnace had to be limited (e.g., to under 8 MW) to keep furnace from self-destructing.